1. Field of the Invention
The present invention relates to a noise canceling method and an apparatus therefor, and in particular to a noise canceling method and an apparatus therefor for faithfully taking out a signal buried in a noise.
Such a noise canceling method and an apparatus therefor has become remarkably necessary in various industrial fields as described in the following.
Power-line carrier modem field which attempts to realize a data transmission at a high speed under circumstances with many noises such as in a power-line carrier;
CATV modem, ADSL modem, VDSL modem, 2.4 G wireless LAN, wireless transmission field, optical transmission field, and the like;
Magnetic disk or optical disk which attempts to realize a high recording density by taking out such a signal as is buried in the noise due to the transmission rate accelerated;
Semiconductor of accelerated multi valued transmission technology;
Voice recognition, image compression, demodulation of bar code scanner, and the like under noisy circumstances.
Hereinafter, the presence of such a noise will be described by taking a power-line carrier modem as an example, while the same applies to the other fields as mentioned above.
In a power system shown in FIG. 20, power in a distribution substation 100 is firstly supplied to a pole transformer 103 through a 6.6 KV high voltage distribution line 102, and is further supplied to a home 105 through a 100 V/200 V low voltage distribution line 104.
Upon performing a power-line carrier communication, an optical fiber (not shown) is set up parallel with the high voltage distribution line 102 between an access node 101 in the distribution substation 100 and a modem set up in the pole transformer. Through the optical fiber, the communication between the modem in the pole transformer 103 and the modem inserted into the convenience outlet connected to an interior distribution line 106 in the home 105 is performed through the 100 V/200 V low voltage distribution line 104.
In this case, as shown in FIGS. 21A–21C, the low voltage distribution line 104 looks like an inductor of 1 μH/m, as shown in FIG. 21B, for a spectrum of a transmission signal TX shown in FIG. 21A, and looks like an inductor of 150 μH if the line length is assumed to be 150 m.
Also, a service wire 107 connected to the low voltage distribution line 104 looks like a capacitor of 75 pF/m, and looks like a capacitor of 0.1125 μF if a 50 m service wire is assumed to be connected to the home 105. Not only the service wire, but also various household electric appliances in the home 105 look like a capacitive load (see FIG. 21B), since the capacitors for canceling the noise are connected to AC 100 V.
Consequently, the portion between the utility pole where the pole transformer 103 is placed and the convenience outlets in the home looks like a low-pass filter (LPF), as shown in FIG. 21B, and a reception signal RX greatly attenuates in a high frequency band, as shown in FIG. 21C. Therefore, when arriving at the terminal side, the high frequency band signal is buried in a noise N in the worst case.
On the other hand, although the loss in the low frequency band is not so large compared with the high frequency band, random noises (white noises) from the household electric appliances such as inverter appliances are extremely large. Therefore, the low frequency band signals are also buried in the noise N as shown in FIG. 21C, and the high-speed data communication can not be put into practice, so that the so has been demanded for a long time.
2. Description of the Related Art
Hereinafter, the prior art technologies for which such solutions have been proposed will be described over three generations.
<1st Generation>
FM modulation method, FSK modulation method, PSK modulation method, and the like said to be resistive to the noise were adopted as a modulation method of a power-line carrier modem. However, since the noise level of the actual power line was high, their practical uses were limited to the applications for a low speed, equal to or less than 1200 bps.
<2nd Generation>
The spread spectrum system was introduced. Since the spread spectrum system was resistive to the noise, it was greatly expected for practical use of the power-line carrier.
However, according to Shannon theory limit, the transmission capacity decreases rapidly when the S/N value is minus or negative (see FIG. 21C) subject to the white noise, and it is theoretically impossible to realize the high-speed transmission. Accordingly, the Shannon limit could not be exceeded, so that the communication of 100 kbps at the maximum could be performed or the communication was disabled in the worst case.
<3rd Generation>
OFDM (Orthogonal Frequency Division Multiplexing) method has appeared. The OFDM method adopts the multi carrier modulation method, and is a technology which avoids the carrier bandwidth with a number of noises without using it. Therefore, a large noise can be avoided, resulting in a prospect of realizing enhanced speed.
However, the inverter built-in rate of the household electric appliances forming the main sources of the noises has been increasing, and the reduction in the high frequency band accompanied with the capacitive load has been also increasing. As a result, although it has been possible to perform a low speed communication depending on circumstances in the prior art technology, it has been impossible to realize a higher speed communication of several Mbps.
Thus, building the inverters in the household electric appliances is a great trend, so that the noise due to the inverters built in the household electric appliances has a tendency to increase more and more hereafter. Also, with the increase of the noise, the capacitive load has a tendency to increase for a noise preventive measure.
Under such circumstances, the idea of avoiding the noise like the solution of the 3rd generation is not enough for the trend. Rather, for the solution of the 4th generation, the attempt of positively facing the noise to cancel the noise, and realize a high-speed data communication is important.
As shown in FIG. 21C, although the noises are little in the high frequency band, the reception signal greatly attenuates by the capacitive load, and becomes lower than the noise level. Although the attenuation of the reception signal is not so large in the low frequency band, the S/N value is still minus due to the noise from the household electric appliances.